That card won't last you 6 months.
There are a few games on the market that you won't be able to play at those resolutions. (I mean, you won't have a playable frame rate). These are some excerpts from them. If you're looking to just play HL2, Doom3, and some Quake 3 based engines... the x850 pro will do fine. But read below...
The first of which is F.E.A.R. FEAR is barely playable at 1024x768 with the x850 (comes in at 30fps max). Don't even think about 1280x960 or above.
Next comes Serious Sam 2 which is barely playable at 1280x960 w/ high settings. (Again, 31fps max). But it's playable at 1024x768
And then comes Black or White 2 which is not even playable at 1024x768 w/ the x850 (21fps max).
Finally there's Age of Empires 3 which is playable @ 1024x768... but barely playable at 1280x1024.
These are games currently on the market... things are only going to get more complicated. F.E.A.R. is arguablly the most advanced game on the market right now. If I were you, I'd look into getting a 6800GT AGP or just save your money for a new motherboard/processor so you can get PCI-e.
So the final answer to your question is no, you won't be able to play nearly every game at 1280x1024 with high settings. You'd be lucky to play half those games @ 800x600 with high settings. Another thing you have consider is your processor. If you're using AGP, you processor is somewhat old. The benchmarks below were all done on an Athlon FX 57 system... fastest you can get. Read the article below:
http://www.tomshardware.com/2005/12/02/vga_charts_viii/page5.html
-mpjesse